A circuit protector having a positive temperature coefficient of resistance is commonly referred to as a PTC device and the specific material that provides the resistance characteristic is commonly referred to as a PTC material or a PTC resistor element. PTC devices are commonly employed in a variety of different electronic devices, such as electric motors, to protect the device against over current and/or excessive temperature conditions.
The PTC device is typically positioned within the current path that supplies power to the protected device such that the current must pass through the resistor element of the PTC device before the current reaches the protected device. Under normal operating temperatures and currents, the resistor element exhibits a relatively low resistance to current and permits the substantially unimpeded flow of current to the protected device. When the current or the environmental temperatures become excessive, the resistivity of the PTC device increases to at least substantially restrict the amount of current delivered to the protected device to prevent the protected device from being damaged.
The resistor element is typically a polymeric resistor element that has a homogeneous mixture of polyolefin material and conductive carbon particles. At normal operating temperatures and currents, the resistor element has a crystalline structure, which provides a low-resistance conductive path device. When excessive temperatures and/or currents are encountered, the resistor element undergoes a phase change (switching action) to an amorphous (non-crystalline) structure and an expansion of the polyolefin. The phase change inhibits conductivity by separating the carbon black particles and results in an increased resistance. The phase change occurs in a very narrow temperature band, resulting in a rapid increase in the resistance of several orders of magnitude. The high resistance state limits current flow to the protected device and protects the device from being damaged by excessive current and/or temperatures. After the excessive temperature and/or current ceases, the resistor element returns to its low-resistance state. The resistor element can be brought to its phase change temperature by self-induced I2R heating or by exposure to an elevated temperature in the surrounding environment.
Even when the PTC device is operating in its low-resistance state under normal operating conditions, the PTC device inhibits, to some extent, current flow to the protected device. Therefore, due to the presence of the PTC device, additional current is required to power the protected device than would otherwise be required in the absence of the PTC device. To conserve energy, it is desirable that the resistance of the resistor element be as low as possible under normal operating currents and temperatures. While the resistivity of conventional PTC devices at standard operating conditions is low enough to provide PTC devices that are suitable for their intended purposes, there is a need for a PTC device having an even lower resistance at normal operating conditions to decrease the amount of current needed to operate the protected device and to therefore conserve energy.
Under normal operating conditions, the overall resistance (Ω) of the PTC device is a function of the resistor element's thickness (t), resistor element area of effective resistance (A) (its length (l) multiplied by its width (w)) and resistivity (ρ), which is a property inherent to the composition of the particular resistor element. Specifically, under normal operating conditions the overall resistance of the PTC device can be calculated by the following equation:Ω=(t/A)(ρ), where A=w*l.The resistor element area of effective resistance is the portion of the resistor element through which current actually passes and is, therefore, the portion of the resistor element that actually provides a resistance to the current. The greater the area of effective resistance (A), the lower the overall resistance (Ω) of the PTC device. Further, it is a well established property that resistors electrically connected in parallel have a lower resistance than resistors connected in series. Therefore, PTC devices having multiple resistor elements in parallel have a lower resistance under normal operating conditions than PTC devices having multiple resistor elements in series.
While PTC devices having a decreased resistance under normal operating conditions can be obtained by increasing the resistor element's area of effective resistance, there exists a competing need to keep the overall dimensions of the PTC device as small as possible to enable the PTC device to be used in applications where space is at a premium. The present invention fulfills the need for a PTC device having a decreased resistance under normal operating conditions by providing PTC devices that each have multiple resistor elements in parallel and an enlarged area of effective resistance as compared to conventional PTC devices. A plurality of the improved PTC devices can be provided together in parallel in a PTC assembly that has a resistance under normal operating conditions that is lower than the resistance of any of the improved PTC devices alone.
An understanding of conventional PTC devices allows one to better appreciate the features of the current invention. FIG. 1 illustrates an exemplary PTC device at 10. The conventional PTC device 10 generally includes a polymeric PTC resistor element 12, a first electrode 14, and a second electrode 16. The resistor element 12 generally includes an upper surface 18, a lower surface 20, a first end 22, and a second end 24. The first electrode 14 has a first portion 26 and a second portion 28. The second electrode 16 generally includes a first portion 30 and a second portion 32. The first portions 26 and 30 and the second portions 28 and 32 are positioned on opposite sides of the resistor element 12.
The first portion 26 of the first electrode 14 is positioned on or closely adjacent to the upper surface 18 of the resistor element 12 and the second portion 28 of the first electrode 14 is positioned on or closely adjacent to the lower surface 20 of the first electrode 14. The first portion 26 and the second portion 28 are electrically connected by a first side electrode 34. The first side electrode 34 spans the thickness of the resistor element 12 at the first end 22.
The first portion 30 of the second electrode 16 is positioned on or closely adjacent to the upper surface 18 of the resistor element 12 and the second portion 32 of the second electrode 16 is positioned on or closely adjacent to the lower surface 20 of the resistor element 12. The first portion 30 and the second portion 32 are electrically connected by a second side electrode 36. The second side electrode 36 spans the width of the resistor element 12 at the second end 24.
The first electrode 14 is positioned such that the first portion 26 opposes the second portion 28 on the opposite side of the PTC element 12. Similarly, the second portion 32 of the second electrode 16 is positioned such that it opposes the first portion 30. The first portion 26 of the first electrode 14 and the second portion 32 of the second electrode 16 extend beyond the second portion 28 and the first portion 30 respectively toward the center of the device 10 and overlap at the center of the device 10. The first electrode 14 and the second electrode 16 are separated by skive marks or gaps 37A and 37B.
With additional reference to FIGS. 2 and 3, when electrical contact is made at any point on the first electrode 14 and the second electrode 16 and electrical current is supplied to the electrodes 14,16, current passes between the electrodes 14,16 through the resistor element 12 in the region ER where the electrodes 12,14 overlap. The region ER is the area of effective resistance of the PTC device 10. As illustrated, the area of effective resistance ER of the resistor element 12 is much smaller than the overall area of the element 12 and the resistance of the device 10 at normal operating conditions is greater than it would be if the area of effective resistance ER of the element 12 was increased. Further, as illustrated in FIG. 3 where the area of effective resistance ER is illustrated in a circuit diagram, the PTC device 10 only has a single area of effective resistance ER, thus causing the PTC device 10 to have a greater resistance under normal operating conditions than it would otherwise have if the resistor element 12 was divided into multiple areas of effective resistance electrically in parallel.
Thus, there is a need for an improved PTC device that exhibits a reduced resistance under normal operating conditions as compared to the conventional PTC devices, such as the PTC device 10.